Temperature compensated pressure differential sensing device with switch actuated by pressure capsule

ABSTRACT

A pressure differential sensing device comprises a sealed housing having mounted therein a gas-filled flexible capsule and a switch operated by expansion of the capsule. The pressure differential sensing device is mounted to a pressurized chamber with the pressure of the chamber communicating with the interior of the housing and surrounding the capsule and monitors the chamber for gas leaks. The gas in the pressurized chamber and in the capsule both respond to variations in temperature in a like manner, and a relatively constant pressure differential is thereby maintained between the interior pressure of the capsule and the pressure of the housing. If gas leaks from the pressurized chamber, the pressure of the chamber decreases relative to the interior pressure of the capsule, and the capsule expands to operate the switch. Thus, the device is temperature compensated. The pressure differential sensing device is calibrated by adjusting the position of the switch relative to the capsule. In one embodiment, the switch is mounted on a threaded collar positioned above the capsule, and the device is calibrated by adjusting the position of the switch. In another embodiment, the capsule is mounted on a first externally threaded collar which is fixed against rotation, and the first collar is mounted to a second interiorly threaded collar which is free for rotation and fixed against axial movement. Rotation of the second collar thereby adjusts the position of the capsule relative to the switch, and such adjustment may be made from outside the housing by providing an access to the second collar through a port in the housing.

United States Patent Meisenheimer, Jr.

[ 1 Nov. 25, 1975 1 TEMPERATURE COMPENSATED PRESSURE DIFFERENTIAL SENSING DEVICE WITH SWITCH ACTUATED BY PRESSURE CAPSULE [76] Inventor: Daniel T. Meisenheimer, Jr., 404

Longmeadow Road, Orange, Conn. 06477 [22] Filed: Apr. 23, 1973 [21] Appl. No; 353,466

[52] U.S. Cl 200/83 S; 116/70; 200/153 T;

[51] Int. Cl. H0111 35/32 [58] Field of Search 73/263, 264, 406, 410,

73/407 R; 340/236, 240, 242; 200/818, 153 T, 83 A, 83 B, 83 D, 83 S, 83 SA, 83 C, 83 R, 83 Y; 116/70; 337/321 [56] References Cited UNITED STATES PATENTS 2,228,533 1/1941 Persons 200/83 A 3,046,369 7/1962 Hicks 200/83 C 3,119,910 1/1914 Meisenheimer, Jr... 200/83 J 3,161,741 12/1964 Wren, Jr. 200/83 D 3,218,593 11/1965 Jonke 200/83 Y 3,419,692 12/1968 Palen 200/153 T 3,444,341 5/1969 Mighton 200/83 S 3,594,753 7/1971 Elenbass... 200/83 D 3,636,289 1/1972 Possell 200/83 B Primary ExaminerGerald P. Tolin Attorney, Agent, or Firm-Wooster, Davis & Cifelli [57] ABSTRACT A pressure differential sensing device comprises a sealed housing having mounted therein a gas-filled flexible capsule and a switch operated by expansion of the capsule. The pressure differential sensing device is mounted to a pressurized chamber with the pressure of the chamber communicating with the interior of the housing and surrounding the capsule and monitors the chamber for gas leaks. The gas in the pressurized chamber and in the capsule both respond to variations in temperature in a like manner, and a relatively constant pressure differential is thereby maintained between the interior pressure of the capsule and the pressure of the housing. If gas leaks from the pressurized chamber, the pressure of the chamber decreases relative to the interior pressure of the capsule, and the capsule expands to operate the switch. Thus, the device is temperature compensated.

The pressure differential sensing device is calibrated by adjusting the position of the switch relative to the capsule. In one embodiment, the switch is mounted on a threaded collar positioned above the capsule, and the device is calibrated by adjusting the position of the switch. In another embodiment, the capsule is mounted on a first externally threaded collar which is fixed against rotation, and the first collar is mounted to a second interiorly threaded collar which is free for rotation and fixed against axial movement. Rotation of the second collar thereby adjusts the position of the capsule relative to the switch, and such adjustment may be made from outside the housing by providing an access to the second collar through a port in the housing.

7 Claims, 7 Drawing Figures US. Patent Nov. 25, 1975 Sheet 2 of2 3,922,515

l I III U W m fifl J00 94 w yz 93/ TEMPERATURE COMPENSATED PRESSURE DIFFERENTIAL SENSING DEVICE WITH SWITCH ACTUATED BY PRESSURE CAPSULE BACKGROUND OF THE INVENTION This invention relates to devices for sensing pressure differentials, and more particularly to a device for sensing relative pressure differentials in a temperature variable environment. This invention further relates to a structure for a temperature compensated pressure differential sensing device adapted for easy calibration.

There are many situations wherein it is desirable to monitor the status ofa pressurized device. One such situation relates to helicopters, and more particularly to helicopter blades which are hollow and filled with a pressurized gas. A loss of or decrease in the pressurization of the blade indicates a defect in the structural integrity of the blade. Defects may be caused by traumatic damage, such as bullet holes, or may be caused by other structural failure, such as fatigue cracks. It is important for the pilot of a helicopter to know immediately if depressurization has occurred, so that he may take appropriate action including landing the helicopter promptly for repairs.

The helicopter blade environment poses problems to a successful pressure monitoring device. A helicopter operates at varying altitudes and consequent wide ranges of temperatures which cause the pressurization of the blade to change. Further, there are vibration and stress loads present in this environment. Also, there is a need for prompt and accurate information concerning pressurization of the blades, and fail-safe operation of the indicator itself is preferable.

Prior art helicopter blade pressure monitoring devices made no attempt to provide an indication of depressurization during flight. These prior art devices were primarily useful for measuring gross deviations in pressure, and had to be read prior to and after flights, and slow depressurization caused by a fatiguecrack might not become apparent on the indicator for a substantial period of time.

SUMMARY OF THE INVENTION The temperature compensated pressure differential sensing device according to the invention herein comprises a closed housing communicating with the interior of a pressurized container the pressure of which is 1 being monitored. Mounted within the closed housing is a gas-filled capsule comprising facing convoluted diaphragms secured together at their peripheries, one side of the capsule having a fill tube extending outwardly therefrom, and the other side of the capsule having a flat, raised surface. A push button actuated switch is mounted within the housing with the push button en gaging the raised flat portion of the capsule wherein the switch is actuated by expansion of the capsule.

The capsule is filled with the same gas as is in the monitored pressurized cavity. Therefore, as the cavity and the capsule are together subjected to varying temperatures, the gas responds in the same way so that the pressure differential between the pressurized cavity and the capsule remains constant, wherein the capsule does not expand and actuate the switch. However, if there is a gas leak from the pressurized cavity, the cavity pressure is thereby reduced and the pressurization of the capsule is then greater in comparison to the pres- 2 surization of the cavity, wherein the capsule expands and actuates the push button switch. The switch is connected to a panel light or other means for indicating that depressurization has occurred.

If the capsule seals fail, the pressure inside and outside the capsule becomes equalized. However, the capsule is constructed and initially pressurized such that it is in a contracted position, and equalization of the pressure because ofa leak in the capsule permits expansion of the capsule and activation of the switch. Although this is a false indication that the pressurized cavity has failed, it is nevertheless a desired indication that the pressure monitoring device is not functioning.

The pressure differential sensing device is calibrated in one of two ways, according to first and second embodiments of the invention. In accordance with the first embodiment, the switch is mounted in the housing on a threaded collar wherein rotation of the collar adjusts the proximity of the switch to the diaphragm. The device is calibrated by altering the position of the switch until the desired indications are achieved for a given pressure and a pressure slightly reduced therefrom.

In the second embodiment of the invention. the

i switch is fixed within the housing, and the capsule is carried on a threaded collar which is free for movement towards and away from the switch but is fixed against rotation. The collar is threadingly engaged with a rotat able sleeve, access to which is provided by an opening through the housing. Thus, turning the sleeve adjusts the position of the capsule relative to the switch, and the capsule is calibrated by pressurizing the housing to a high pressure and thereafter bleeding the pressure to the desired alarm" pressure, while monitoring the switch condition. The sleeve is rotated until the desired indication is received at the alarm pressure.

OBJECTS OF THE INVENTION It is a principal object of the invention to provide a pressure differential sensing device for detecting leaks in pressurized containers.

It is an additional object of the invention to provide a pressure differential sensing device for detecting leaks in pressurized containers wherein the containers are subjected to varying temperatures.

It is a furuther object of the invention to provide a pressure differential sensing device which incorporates means for remote indication of the sensed condition.

It is another object of the invention to provide a rug ged temperature compensated pressure differential sensing device suitable for aircraft and other environments requiring high reliability and durability.

It is still a further object of the invention to provide a temperature compensated pressure differential sensing device the calibration of which may be externally ad justed.

These and other objects of the invention will in part be obvious and will in part appear in the following description of the preferred embodiments taken together with the drawings.

DRAWINGS FIG. I is a top plan view of a temperature compensated pressure differential sensing device according to the invention herein:

FIG. 2 is a side elevation view of the device of FIG. 1;

FIG. 3 is a sectional view of the device of FIG. I taken along the lines 3-3 of FIG. 2;

FIG. 4 is a perspectiw iew of a prcssuri/cd capsule and mounting collar incorporated in the device of FIG. I:

FIG. 5 is a side elevation view of another embodiment of a temperature compensated pressure differential sensing device according to the invention herein;

FIG. 6 is a side vievt partially in elevation and partially in section of the device of FIG. 5; and

FIG 7 is a sectional view of the device of FIG. 5 taken along the lines 77 of FIG. 6.

The same reference numbers refer to the same elements throughout the various figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment 10 of a pressure differential sensing device according to the invention herein is shown in FIGS. 1-4. It generally comprises a housing II, a filled capsule subassembly I2, a switch subassembly 13, a wire-header subassembly I4, and a closure cap 15.

The housing 11 comprises a hexagonal base having a cylindrical portion 21 extending upwardly therefrom, the base 20 and the cylindrical portion 21 to gether defining a cavity for receiving the various subassemblies.

The interior of the cylindrical portion 21 of housing II is internally threaded at 22 beginning above the hexagonal base 20 and extending upwardly to a shoulder 23. Above shoulder 23 is a larger diameter smooth interior portion 24 extending upwardly from the shoulder 23 to a second shoulder 25. The remaining interior surface of cylindrical portion 21 is internally threaded at 26. A slot 27 extends downwardly from the top of the cylindrical portion 21 and terminates within the span of the smooth interior surface 24. Extending downwardly from the hexagonal base 20 is a shank 30 which is externally threaded at 31 and 32 for securing the pressure differential sensing device 10 to a pressurized chamber (not shown), The shank 30 further includes a central bore 33 connecting the tip of the shank and the interior of the housing I]. A filter 34 may be provided within the bore 33 at the tip of the shank to prevent particulate matter from contaminating the interior of the hous- The filled capsule subassembly I2 is shown isolated in FIG. 4 and is also shown in FIG. 3. It comprises a capsule 40 which is fabricated from two convoluted diaphragms joined together along their peripheral edges. Extending upward from the top of capsule 40 is a raised flat surface 41, and extending downward from the underside of capsule 40 is an elongated fill stem 42. The fill stem 42 extends through a threaded collar 43 having a hexagonal head portion 44, and the stem 42 of capsule 40 is secured therein by solder with the collar 43 positioned adjacent to the underside of capsule 40.

The capsule 40 is filled with a gas to a preselected pressure. This is accomplished by heating the capsule to approximately 200 F. in order to vaporize any moisture therein, and thereafter evacuating the capsule through stem 42. The capsule is then backfilled with gas through stem 42 to a pressure substantially greater than the preselected pressure. the temperature of the capsule being maintained during backfilling. The evacuating and backfilling steps are repeated several times to insure purging of all air and moisture from the capsule,

The capsule, backfilled with gas at the substantially greater pressure. is then cooled to room temperature.

The pressure within the capsule is then bled to the desired pressure. the desired pressure being determined as the preselected pressure compensated for existing temperature and barometric deviations. The stem 42 is then crimped and soldered, as indicated at 45, to seal the gas within capsule 40.

The capsule 40 is preferably filled with the same gas as is in the pressurized chamber to which the pressure differential sensing device is mounted. In the case of helicopter rotor blades, the gas is normally a mixture of percent nitrogen and l() percent helium, the helium functioning as a tracer element permitting the use of spectroscopy techniques in locating gas leaks. By using the same gas for filling the capsule as is used in a pressurized chamber, the gas in both the capsule and the chamber responds to temperature changes in a like manner according to Boyle Charles Laws.

In an assembled pressure differential sensing device mounted to a pressurized chamber, the capsule 40 is surrounded by gas from the chamber. Thus there are three main forces acting on the capsule: first, the extcrnal gas pressure of the chamber; second, the internal gas pressure of the capsule; and third, the forces inherent in the capsule itself when it is held in a position other than its free position by the first two forces.

The basic operating principle of the pressure differential sensing device disclosed herein is that a loss of gas from the pressurized chamber to which it is mounted decreases the pressure outside capsule 40, permitting capsule 40 to expand and actuate a signalling device. The chamber pressure varies with temperature, and if an evacuated capsule were used, the device would signal in response to lowered pressure due to decreased temperature or in response to lowered pressure due to loss of gas. The gas within the capsule compensates for the variations in temperature by also respond ing thereto.

A temperature-pressure curve is established for the chamber assuming no loss of gas therefrom. This curve normally discloses a substantial linear relationship between temperature and pressure.

It has been found that selection of the backtill pres' sure of the capsule determines the slope of response of the capsule, and that a trial and error" empirical determination of the desired backfill pressure can be made such that the capsule will track the temperaturc-pressure curve of the chamber, i.e., the capsule responds only if the pressure of the chamber is below the pressure expected for the given temperature. The desired backfill pressure is necessarily an empirical value because the variety of parameters associated with the capsule. including the inherent spring forces and variable volume of the capsule itself, make calculation ofa value extremely difficult.

The normal pressurization of gas in a helicopter rotor blade is approximately 15 psi. above sea level pressure, which is substantially higher than the initial pressurization of capsule 40. Capsule 40 is thereby normally compressed from its free, relaxed position, which provides a fail safe mode, as will be more fully discussed below.

After initial pressurization of capsule 40 has been accomplished, the capsule 40 and its associated threaded collar 43 are positioned in a mounting collar 46 by turning collar 43 into a central internally threaded bore therein. and the capsule is secured in the mounting collar 46 by means of a nut 47 threaded onto the lower portion of collar 43. This structure comprises the filled capsule subassembly 12. The mounting collar 46 is externally threaded, the threads mating with threads 22 of housing II, and the filled capsule subassembly I2 is mounted in the housing 11 by turning mounting collar 46 into threads 22 of housing 11 until the collar bottoms on the base 20, as shown in FIG. 3. Two holes 48 and 49 provide communication between the bore 33 and the portion of the interior of housing 11 above the mounting collar 46.

The fill tube 42 extends into bore 33 when the cap sule is positioned in housing 11, and access to the fill tube 42 is available through bore 33. Reevacuation and rebackfilling of the capsule is provided by the length of capillary. Also, one may crimp all or a portion of the fill tube to decrease the volume of the capsule and fill tube, should it be desired to do so.

The switch subassembly 13, best seen in FIG. 3, comprises a threaded cylindrical collar 50 having mounted thereon a cross piece 51 to which a switch 52 is secured by means of screws 53 and 54. The switch is a snapaction, single throw, double pole switch operated by a push button 55 which depends downwardly from the switchbody in the orientation in which the switch is mounted. The preferred switch is identified by specification No. MS25085-l.

The switch subassembly 13 is installed in the housing 11 by screwing the threaded collar 50 into the threads 22, and the position of the collar 50 establishes the proximity of push button 55 to the top surface 4] of the capsule 40. The position of push button 55 determines the amount of expansion of the capsule 40 which is necessary to operate the switch. Thus, the pressure differential sensing device is calibrated to a certain temperature adjusted pressure by means of adjusting threaded collar 50. This is accomplished by temporarily sealing the upper end of housing I1 and pressurizing the housing to a high pressure. In preparing a sensing device for installation in a helicopter blade where the normal pressure is p.s.i. above sea level pressure and the desired alarm pressure may be 5 p.s.i. above sea level pressure at 70 F., the initial pressurization may be lO-l5 p.s.i. above sea level pressure. The capsule should be sufficiently compressed with respect to the switch at the high housing pressure that the switch is not actuated. The pressure is then bled until the switch is operated. and the position of the collar 50 is adjusted until the switch is operated at the proper alarm pressure. Care must be taken during this procedure to maintain the temperature of the capsule and housing at la constant level, and to adjust the switch for operation at the proper alarm pressure for that temperature.

The collar 50 is locked in position by means of a screw 56 extending downward and butting against collar 46 once the calibrated position of the switch is achieved.

The wire-header subassembly 14 comprises a cylindrical member 60 having an outside diameter approximately equal to the inside diameter of the smooth inside surface 24 ofthe housing 11. The cylindrical member 60 has a smaller diameter cylindrical portion 61 extending upwardly therefrom, and plate 62 is provided therebetween. Terminals 63 and 64 extend through the plate 62 wherein they are exposed adjacent to switch 52 for connection to the terminals thereof. Above plate 62 the terminals 63 and 64 are connected to wire leads 65 and 66, surrounded by sheathing 67, which extend out of the upper cylindrical portion 61 at a port 68. Upon assembly ofleads 65 and 66 to the upper portions of terminals 63 and 64, the space above plate 62 and surrounded by cylindrical portion 61 is sealed with a potting compound, as indicated at 69.

A rubber grommet 74 is provided around sheathing 67 for sealing the slot 27. An elongated rubber grommet may be used, or a round grommet as shown may be used with the area of a slot above the grommet 74 being potted or otherwise sealed against moisture intromis sion or the like.

The outer periphery of the cylindrical portion 60 of the wire-header subassembly 14 is provided with pe ripheral grooves 70 and 71 in which are carried two 0- ring seals 72 and 73. The wire-header subassembly 14 is slid into the housing 11 and the O-rings provide a seal between the cylindrical member 60 and the smooth inner surface 24 of the housing ll. Therefore, the wireheader subassembly 14 together with the housing 11 defines a closed cavity surrounding the capsule 40, the cavity communicating with the tip of shank 30 through bore 33.

Assembly of the pressure differential sensing device is completed by screwing a threaded cap 15 in to the internal threads 26 of housing 11, and securing cap 15 against rotation by means of screws 75 and 76. The cap 15 provides a completed assembly restraining the wireheader subassembly 14 from being blown out or moved upward by pressure, and the cap 15 further provides an environmental seal for the potted wire leads.

The pressure differential sensing device 10 described above is mounted to a pressurized chamber such as a helicopter blade by providing a threaded opening therein for receiving the threaded shank 30. The bore 33 thereby communicates with the interior of the chamber, and the pressure thereof is equalized with the pressure of the interior of the housing 11 surrounding the capsule 40. As the chamber and the capsule are subjected to variations in temperature, variations of their internal pressures occur. However, the overall forces on the capsule from the pressures are maintained in balance, and the capsule does not expand to operate switch 52.

In the event that the pressurized chamber loses some of its gas, the pressure of the chamber is reduced with respect to the pressure of capsule 40 and the capsule expands, thereby depressing the push button 55 to operate the switch 52. When the switch is operated, an indicator such as a panel light may be illuminated thereby indicating at a remote position that gas loss and depressurization has occurred. The actual pressures at the time of a leak may vary widely depending on temperature, but such actual pressures do not affect operation of the device. The device is sensitive only to changes in the relative pressures of the chamber and capsule.

If the integrity of the capsule 40 itself should fail, the pressure inside capsule 40 becomes equalized with the pressure in the chamber. If the initial pressure of capsule 40 was below the pressure of the chamber, the capsule 40 was thereby normally maintained in a contracted or compressed condition. Equalization of the pressure within the capsule permits the capsule to expand, thereby depressing push button 55 and indicating gas loss or depressurization of the chamber. Although in this situation the chamber is actually not depressurized and the signal is false, it is nevertheless helpful for this signal to occur to indicate that the monitoring system is not in proper working order.

Referring now to FIGS. 5-7 a second embodiment of a temperature compensated pressure differential sensing device according to the invention herein is shown. The operating principle of the pressure differ ential sensing device 80 is the same as in the first embodiment l0, i.e., a pressurized capsule 40 is positioned within a pressurized housing adjacent to a switch 52 wherein changes in the pressure differential between the capsule and a chamber in communication with the housing cause operation ofthe switch. However, in the embodiment 80 the switch is held stationary and the capsule is movable relative to the switch for calibrating the device. Further, the embodiment 80 includes means for moving the capsule relative to the switch from outside the housing and after final assembly of the device.

Referring now to FIGS. and 7, the pressure differential sensing device 80 comprises a generally cylindrical housing 81 through which a port 82 is provided. The switch subassembly 13, the wire-header subassembly 14 including the associated sealing devices and potting compounds, and the cap are the same as described above with respect to the first embodiment 10. During assembly of these items into housing 81, the position of switch subassembly 13 is preselected within a desired range. and the switch subassembly is locked at that position.

The housing 81 further comprises an annular flange 84 extending inwardly from the housing wall and forming on its underside a shoulder 85. The interior of the housing further comprises a smooth portion 86, which is intersected by the port 82. A shoulder 87 and a threaded portion 88 complete the interior of the housing 81.

The housing 11 is provided with a bottom plate 90 which is screwed into threads 88 to abut against shoulder 87. Extending downwardly from the bottom plate 90 is a shank 92 having threads 93 and 94 on an outside surface thereof. The shank 93 has a hollow bore 95 passing therethrough. A filter 34 may be provided at the end of the bore. The bottom plate 90 is provided with a hexagonal head 91 which facilitates turning the bottom plate into the housing and also facilitates mounting the assembled pressure differential sensing device 80 into a helicopter blade or other chamber.

Upstanding from the bottom plate 90 are two pins 96 and 97 which pass through openings in a collar 98 and secure the collar 98 against rotation. A capsule 40 as shown in FIG. 4 is evacuated and backfilled with gas as described above. The capsule is mounted in a threaded collar 83, and is attached to the collar 98 by means of collar 83.

The threaded collar 98 is surrounded by an internally threaded annular collar 99 which is constrained between shoulder 85 and bottom plate 90. The outside surface of collar 99 mates with the smooth inner surface 86 of housing 81, wherein collar 99 is free for rota tion.

Rotation of collar 99 causes collar 98 to be moved either upwardly or downwardly on the mating threads between collars 98 and 99, the direction of movement depending on the direction of such rotation. Movement of collar 98 in turn adjusts the position of capsule 40 relative to the push button 55 of switch 52 for calibrating the pressure differential sensing device 80 to the desired alarm pressure.

Rotation of collar 99 may be accomplished from outside housing 11 by engaging one of the indentations 100 provided in the outer surface of c illar 99 through port 82, as best seen in FIGS. 5 and 7.

Collar 99 is provided with a peripheral groove I01 and O-ring seal 102 for maintaining and airtight seal between the housing wall and the collar. Bottom plate 91 is provided with an annular groove 103 carrying an O-ring seal 104 to complete sealing of the interior of the housing 81, except for the pressure com munication path provided by bore 95.

The pressure differential sensing device is mounted to a pressurized cavity via threaded shank 92, wherein the pressure of the cavity communicates with the interior of the housing through bore 95. Operation of the embodiment 80 is similar to the operation of the embodiment 10. A loss of gas and the resultant depressurization in the helicopter blade or other chamber to which the device is attached permits capsule 40 to expand and operate switch 52.

The advantage of the embodiment 80 of the pressure differential sensing device is the ease of calibration. Calibration of the device can be accomplished after it has been installed to a chamber, calibration being accomplished by pressurizing the chamber to the desired alarm pressure and rotating collar 99 to move the capsule 40 upward until switch S2 is operated. The chamber may then be pressurized to its normal pressure, which will compress capsule 40 such that switch 52 is not operated.

Calibration of the embodiment 80 at the time of manufacture is also more easily accomplished. Particular advantages include the ability to calibrate after final assembly of the housing, and the ability to calibrate without touching the housing, which introduces unwanted temperature variations at the time of calibration.

The pressure differential sensing devices disclosed above are well suited for use in monitoring the internal pressure of helicopter blades, and are also well suited for monitoring the pressurization of any other pressurized container. The devices are rugged and highly reliable.

It will be appreciated that although the pressure differential sensing devices disclosed above provide an expandable capsule for operating a push button switch, other signal devices may be operated by the capsule. Such other devices may include potentiometers, variable transformers, or other suitable devices. It should, therefore, be understood that the foregoing description is directed to a preferred embodiment of the invention and is representative only, and various departures from the preferred embodiments shown and described may be made without departing from the spirit and scope of the invention.

What l claim is:

l. A pressure differential sensing device comprising:

a. a housing having a base, a cylindrical body portion extending upwardly therefrom, and a shank portion extending downwardly therefrom, the shank portion having a bore formed therethrough;

b. a mechanically operated switch including an actuating member mounted in the housing;

c. a flexible gas filled sealed capsule mounted within the housing, said capsule mounted to a first externally threaded collar which is fixed against rotation and free for axial movement with respect to the switch actuating member, said first collar mounted in mating interior threads of a second collar, said second collar constrained within the housing against movement along the axis of its interior threads and free for rotation about said axis, wherein rotation of said second collar causes movement of said first collar and the capsule mounted thereto for adjustably positioning the capsule relative to the actuating member of the switch; d. signal means operated by the switch; and

e. means sealing the housing wherein the interior of the housing comprises a sealed cavity communicating with the exterior of the housing through the bore in the mounting shank;

wherein the device is attached to a pressurized chamber by means of the shank and wherein the initial internal pressurization of the capsule is chosen such that the capsule size remains substantially constant throughout a range of temperature variations causing pressure increases and decreases in both the capsule and the chamber, whereby a loss of gas from the chamber results in a decrease of the gas pressure of the chamber relative to the internal gas pressure of the capsule thereby permitting the capsule to expand, the expansion of the capsule operating the switch.

2. The pressure differential sensing device as defined in claim 1 wherein the capsule comprises two convoluted diaphragms joined together along their peripheral edges.

3. The pressure differential sensing device as defined in claim 1 wherein the capsule and the chamber contain the same gas.

4. The pressure differential sensing device as defined in claim I wherein the housing defines an opening providing access to the second collar for rotating the second collar from outside the housing.

5. The presure differential sensing device as defined in claim I wherein the first collar is fixed against rotation by at least one pin fixed to the housing and extending into an opening in said first collar, the pin and the opening being axially aligned with the direction of permissible movement of the first collar.

6. The pressure differential sensing device as defined in claim 1 wherein the signal means comprises a lamp positioned remotely from the housing and connected 10 thereto by wire leads, the lamp being operated by the switch.

7. A pressure differential sensing device comprising:

a. a housing having a base, a cylindrical body portion extending upwardly therefrom, and a shank portion extending downwardly therefrom, the shank portion having a bore formed therethrough;

b. a flexible gas filled sealed capsule mounted within the housing;

c. a mechanically operated switch including an actuating member, said switch mounted to a first externally threaded collar fixed against rotation and free for axial movement with respect to the capsule, said first collar threaded into mating interior threads of a second collar, said second collar being constrained within the housing against movement along the axis of its interior threads and free for rotation about said axis, wherein rotation of the second collar causes movement of said first collar and the switch mounted thereto for adjustably positioning the switch relative to the capsule;

cl. signal means operated by the switch, and

e. means sealing the housing wherein the interior of the housing comprises a sealed cavity communicating with the exterior of the housing through the bore in the mounting shank;

wherein the device is attached to a pressurized chamber by means of the shank and wherein the initial internal pressurization of the capsule is chosen such that the capsule size remains substantially constant throughout a range of temperature variations causing pressure increases and decreases in both the capsule and the chamber, whereby a loss of gas from the chamber results in a decrease of the gas pressure of the chamber relative to the internal gas pressure of the capsule thereby permitting the capsule to expand, the expansion of the capsule operating the switch. 

1. A pressure differential sensing device comprising: a. a housing having a base, a cylindrical body portion extending upwardly therefrom, and a shank portion extending downwardly therefrom, the shank portion having a bore formed therethrough; b. a mechanically operated switch including an actuating member mounted in the housing; c. a flexible gas filled sealed capsule mounted within the housing, said capsule mounted to a first externally threaded collar which is fixed against rotation and free for axial movement with respect to the switch actuating member, said first collar mounted in mating interior threads of a second collar, said second collar constrained within the housing against movement along the axis of its interior threads and free for rotation about said axis, wherein rotation of said second collar causes movement of said first collar and the capsule mounted thereto for adjustably positioning the capsule relative to the actuating member of the switch; d. signal means operated by the switch; and e. means sealing the housing wherein the interior of the housing comprises a sealed cavity communicating with the exterior of the housing through the bore in the mounting shank; wherein the device is attached to a pressurized chamber by means of the shank and wherein the initial internal pressurization of the capsule is chosen such that the capsule size remains substantially constant throughout a range of temperature variations causing pressure increases and decreases in both the capsule and the chamber, whereby a loss of gas from the chamber results in a decrease of the gas pressure of the chamber relative to the internal gas pressure of the capsule thereby permitting the capsule to expand, the expansion of the capsule operating the switch.
 2. The pressure differential sensing device as defined in claim 1 wherein the capsule comprises two convoluted diaphragms joined together along their peripheral edges.
 3. The pressure differential sensing device as defined in claim 1 wherein the capsule and the chamber contain the same gas.
 4. The pressure differential sensing device as defined in claim 1 wherein the housing defines an opening providing access to the second collar for rotating the second collar from outside the housing.
 5. The presure differential sensing device as defined in claim 1 wherein the first collar is fixed against rotation by at least one pin fixed to the housing and extending into an opening in said first collar, the pin and the opening being axially aligned with the direction of permissible movement of the first collar.
 6. The pressure differential sensing device as defined in claim 1 wherein the signal means comprises a lamp positioned remotely from the housing and connected thereto by wire leads, the lamp being operated by the switch.
 7. A pressure differential sensing device comprising: a. a housing having a base, a cylindriCal body portion extending upwardly therefrom, and a shank portion extending downwardly therefrom, the shank portion having a bore formed therethrough; b. a flexible gas filled sealed capsule mounted within the housing; c. a mechanically operated switch including an actuating member, said switch mounted to a first externally threaded collar fixed against rotation and free for axial movement with respect to the capsule, said first collar threaded into mating interior threads of a second collar, said second collar being constrained within the housing against movement along the axis of its interior threads and free for rotation about said axis, wherein rotation of the second collar causes movement of said first collar and the switch mounted thereto for adjustably positioning the switch relative to the capsule; d. signal means operated by the switch; and e. means sealing the housing wherein the interior of the housing comprises a sealed cavity communicating with the exterior of the housing through the bore in the mounting shank; wherein the device is attached to a pressurized chamber by means of the shank and wherein the initial internal pressurization of the capsule is chosen such that the capsule size remains substantially constant throughout a range of temperature variations causing pressure increases and decreases in both the capsule and the chamber, whereby a loss of gas from the chamber results in a decrease of the gas pressure of the chamber relative to the internal gas pressure of the capsule thereby permitting the capsule to expand, the expansion of the capsule operating the switch. 